The present invention relates to end-pumped solid-state lasers, and more particularly, to lasers which are pumped by one or more semiconductor laser diodes. In general, such systems have required a fair degree of ingenuity or purposeful design to inject the laser diode output light into the solid-state rod or crystal in a manner to obtain a useful output. Generally, for end-pumped lasers, this has meant selecting optics that match the pump light to the TEM.sub.oo crystal mode, or entailed using crystals of very high absorbance to limit the region in which pump light is concentrated; it has further required a number of elements to be precisely adjusted and aligned to achieve effective operation.
Descriptions in patents tend to be written at an early stage of the development of a technology, and may not accurately reflect either the technology, or the true scope of results achieved by the techniques sought to be patented. However, a brief review of patents in this area offers some insight as to constructions which the patentees believe to be promising or useful for effecting laser pumping systems of this type.
U.S. Pat. No. 3,982,201 of Rosenkrantz claims an end-pumped solid-state laser using an array of pump light emitting semiconductor diode lasers arranged to direct their light onto an end of the solid-state laser. It suggests that the laser diode array could be pulsed at such a rate as to induce continuous wave (CW) operation of the solid-state laser. The array is a physical configuration such that diode light enters the Nd:YAG laser crystal through an end face.
U.S. Pat. No. 4,847,851 of Dixon claims end pumping a solid-state laser by "butt-coupling" a laser diode to one end of the rod and using a "very short absorption length . . . less than 500 micrometers" so as to absorb all of the diode pump light within the TEM.sub.oo mode volume. It further has a rear laser mirror coated directly onto a face of the diode.
U.S. Pat. No. 4,710,940 of Sipes claims a configuration in which an array of laser diodes are positioned to pump the laser medium in the direction of the axis, and requires a means for concentrating diode light inside the lasing volume. This is illustrated as a series of lenses which tailor the light from diode array to provide a focused spot inside the laser rod that overlaps the TEM.sub.oo mode volume of the laser.
U.S. Pat. No. 4,653,056 of Baer, et al., claims a laser diode array secured in a housing behind and in optical alignment with a laser rod. The preferred construction of this device also uses a series of lenses to provide focused spots inside the laser rod in the mode volume.
In summary, these prior art and pumping technologies have been restricted to laser diode arrays mounted in optical alignment to the laser rod, within a laser head. Generally, the diode array is a linear array with astigmatic properties and may require fairly sophisticated optical arrangements to bring its output into a symmetrical, semi-circular output suitable for end pumping (see for example, the papers of Zhou, et al. (1985), Berger, et al., (1988), and Oka and Kubota (1988)).
In addition to these constructions, various researchers have suggested using one or more optical fibers to couple pump light to a laser.
Recently, a new type of laser diode source has emerged which produces a large output at a single aperture by coupling many single diodes through separate optical fibers which are then bundled together to form a single, circular emission aperture. The entire assembly is a self-contained package with a short metal ferrule protruding from one side; the ferrule collects the individual fibers into a bundle of circular cross-section. This bundle is coupled via a permanently bonded interface into an SMA connector which couples the output light into a single-core fiber. This homogenizes the diode outputs, and produces an output of Gaussian circular intensity profile at the fiber output. This output is well matched for end-pumping the TEM.sub.oo mode of a crystal. since that mode also has a Gaussian intensity profile. At present, this type of multi-fiber-coupled and single-fiber-output high power CW diode array is available from two commercial suppliers, SDL, Inc., and Opto Power Corp. Typical output powers of these devices approach or exceed six watts. Because of this high output power, one would expect these diode packages to be useful as pump lasers for various laser media. Indeed. Baer, et al., in U.S. Pat. No. 4,723,257 have previously suggested using a fiber-coupled, laser-diode source to power a remote laser head, since this would allow the head to be entirely free of electronics and, thus, to have a relatively simple and robust architecture. The fiber output of these current laser diode arrays would appear to be amenable to positioning and focusing so as to implement at least some of the particular pumping configurations described in one or more of the above patents. However, as a trade-off for the ease of handling obtained by having a diode source with fibers already attached and bundled, one must settle for a permanently attached and aligned high-power single fiber jumper. One might expect that highly specialized optics, for example, relatively large aperture long focal length optics, would be required to match the output to an end-pumped crystal. Moreover, the pumping qualities of such a strong source with its beam derived of multiple small fiber output beams are not fully explored.